The term x86 denotes a family of backward compatible instruction set architectures based on the Intel 8086 CPU. From Wikipedia: http://en.wikipedia.org/wiki/X86
The term x86 denotes a family of backward compatible instruction set architectures based on the Intel 8086 CPU. The 8086 was introduced in 1978 as a fully 16-bit extension of Intel's 8-bit based 8080 microprocessor, with memory segmentation as a solution for addressing more memory than can be covered by a plain 16-bit address. The term x86 derived from the fact that early successors to the 8086 also had names ending in "86".
Many additions and extensions have been added to the x86 instruction set over the years, almost consistently with full backward compatibility. The architecture has been implemented in processors from Intel, Cyrix, AMD, VIA and many other companies; there are also open implementations, such as the Zet SoC platform.
The term is not synonymous with IBM PC compatibility as this implies a multitude of other computer hardware; embedded systems as well as general-purpose computers used x86 chips before the PC-compatible market started, some of them before the IBM PC itself.
From Wikipedia: x86
For ABI documents, assembly reference manuals, guides, tutorials, and optimization / performance stuff, see Stack Overflow's x86 tag wiki